Ultrasound is widely used for imaging of blood vessels because it is non-invasive, real-time, and relatively inexpensive. Reliable quantitative evaluation of blood vessels plays a pivotal role in cardiovascular disease diagnosis and follow-up intervention to avoid progression to life or limb-threatening stages. These studies require accurate vessel measurement for size analysis and registration between serial studies for monitoring disease progression before and/or after vascular repair. Sites of particular interest are (1.) carotid arteries for risk of stroke, (2.) lower-limb bypass grafts for risk of limb loss, and (3.) abdominal aortic aneurysms for risk of rupture. Newly developed endo-vascular treatments further demand highly accurate 3D reconstructions of vessels for follow-up to assure success of the procedure or to evaluate the efficacy of the devices. A major constraint to 3D vascular imaging is the imprecision and mechanical restrictions of positioning tools. Magnetic tracking overcomes line-of-sight and mechanical restrictions, but has range limitations and distortion caused by nearby metal. Optical technology is precise but suffers from occlusion, high cost, and bulkiness. For clinical acceptability, a new approach is needed to overcome limitations while allowing precise measurment of vascular structures and/or changes. The Phase I goal will prove the feasibility of a new, hybrid (magnetic-optical) tracking technology for use in 3D vascular imaging. Accuracy of 0.5 mm/0.5 degree is the first design goal. Phase II product goals will be: (1.) Accurate (0.1mm/0.07degree) measurement of a sensor attached to an ultrasound scanhead, (2.) freedom to track the scanhead the length of an adult limb of a patient lying on a standard medical procedural table, (3.) no data impairment due to metallic distortion or noise interference. Full development will enable clinicians to quantitatively analyze 3D reconstructions as a precise means of assessing morphological changes over time, compared to current 2D slice-by-slice delineation and interpretation of vessel boundaries. The new technology will also further the development of new imageguided techniques for use in minimally-invasive procedures.